1. Field of the Invention
The invention relates to the field of motionless mixers and more particularly to the application of motionless mixers to the mixing of gas in a liquid, such as the oxygenation of waste water in waste water treatment systems.
2. Description of the Prior Art
Motionless mixers generally consist of a helical blade or vane which is fixed in a pipe or tube through which a liquid flows in order to mix the components of the liquid. They are referred to as motionless mixers because they have no moving parts. As the fluid flows through the motionless mixer, it is divided into two streams which are rotated about the central axis of the pipe, resulting in a mixture of the fluid once it passes through the motionless mixer. For example, see U.S. Pat. No. 4,466,741 in the name of Kojima, entitled "Mixing Element and Motionless Mixer", issued Aug. 21, 1984 which describes the prior art in the field of motionless mixers. The mixing effect is partly achieved through an increase in the area of contact between the materials to be mixed.
In many applications it is necessary to dissolve gases in a stream of liquid. For example, in sewage treatment it is desirable to dissolve oxygen in a flow of waste water. Similarly, the pulp and paper process involves at one stage a dissolving of oxygen in a stream of liquid. The wine-making process also involves the dissolution of nitrogen in a stream of wine. Up to the present this method of dissolving gases in liquid has involved simply injecting gas into the stream of liquid, or pumping the gas through the liquid, thereby dissolving the gas at atmospheric pressure. The rate of dissolution of gas in the liquid is lower than would be the case if the liquid were under a higher pressure.
Motionless mixers have been used in the mixing of gases and liquid, such as the oxygenation of water. Motionless mixers have also been used in waste water treatment systems, both to mix solid or liquid additives, to assist in the flocculation process, and to dissolve gases such as oxygen in the water. The problem in such applications has been to maximize the oxygen transfer and absorption efficiency in the waste water. Compressed oxygen is an expensive commodity. Even with the addition of a standard motionless mixer, much of the oxygen remains in the gaseous state as bubbles in the liquid and is eventually lost to the air, rather than being dissolved in the water to increase the beneficial activity of bacteria.